1. Field of the Invention
This invention relates to resistance welding apparatus of a kind in which contact pressure, between a pair of metal surfaces joined, is imposed through the medium of an intermediate electrode wire guided by an electrode wheel on each side of the metal surfaces.
This kind of welding apparatus is particularly useful for welding steel sheet materials that have a metallic coating such as zinc, tin or chromium bearing layers (TFS) because the intermediate wire electrode is progressed around the electrode wheels to present a clean contact surface during welding and then take away any contaminating oxides or fused coating.
2. Description of the Invention
Particularly successful machines are sold by SOUDRONIC AG of Switzerland and described in British Pat. Nos. 910,216, 1,124,885 and 1,426,356 to which the reader is directed for further details. This kind of apparatus is widely used in the can making industry in which rectangular blanks of tinplate of TFS are roll formed to a cylinder having an overlapped side seam which is mash welded to make a can body.
European patent application No. 0041893 considers the problem arising when relatively small can bodies are being welded, namely the fact that the space inside the can body can only accommodate an electrode wheel that is smaller than the outside electrode wheel so that the difference in amplitude between the ridges and hollows along the weld, as created by each weld pulse, is accentuated on the interior surface of the weld. An internal electrode comprising support means to support an internal welding wheel which preferably rotates and a support roll is so arranged that the electrode wire passes across the welding wheel at which welding takes place, and continues its passage to the support roll while remaining in contact with the weld to improve the geometry of the weld and therefore its homogeneity.
French Patent published application No. 2553320 considers the distribution of energy arising in a lap welding process when the external electrode wheel is bigger than the internal welding wheel. The Applicants have surmised that the asymmetry of the contact surfaces may lead to problems with the quality of the spot welds, because energy released at the interface between the electrode with a smaller radius of curvature and the edge which it contacts is greater than the energy released at the interface between the second edge of the body and the electrode with a greater radius of curvature. As the material thickness t of each metal margin overlapped is reduced in the finished weld to a total wall thickness of about 1.5 t, this model is reasonable and leads to the conclusion that the maximum heating effect will be off-centre from the can metal to can metal contact at which it is needed. Accordingly, it is proposed in French Patent application No. 8316357 that the resistance of the coatings on the sheet metal to be welded should be selected to compensate for the difference between the internal and external contact areas.
The prior art apparatus uses costly mercury contacts to deliver electricity to the rotating electrode wheels. As one of the wheels is inside the can bodies being made there is a risk that any leakage of mercury will seriously contaminate a can interior.